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smithy123_gw

running a generator in paralell with an inverter

smithy123
13 years ago

does anybody know a way to safely run a generator in paralell with an inverter? i have the cpi 1575 inverter. the problem is, the battery doesnt last long, even with the engine running. it is not my truck, i am using this at a parade and use it supported by t&b ty rap cablt ties. i do not want a transfer switch, nor do i want them wired with 2 male plgs in paralell.

Comments (33)

  • Ron Natalie
    13 years ago

    If the inverter was up to it, you could. The cheapo car ones like you have are NOT up to that. Moer expensive ones as used with solar systems can match the frequency of the incoming line or generator.

    Congratulations on not making the suicide double male plug.

    The only kludge I can imagine was to take the generator out to make 12V to feed the inverter.

    Of course, if you just had a square D QO panel, all would be solved.

  • smithy123
    Original Author
    13 years ago

    i know if i switched the power to dc using bridge rectifiers i could run them in paralell, but then i would not be able to run a motor or transformer supplied appliance.

    9

  • Ron Natalie
    13 years ago

    It would work fine as long as you connected it to a QO panel and made sure you used a GreenLee screwdriver to make the connections.

    If your concern is that the battery goes dead, runt he generator into a DC supply that you can parallel with the battery and plug everything into the inverter's DC input. This is exactly how commercial UPS systems work.

  • smithy123
    Original Author
    13 years ago

    the dc power supply's output power fluctuates too much, but i can try it next year.
    btw, where did the screwdriver thing come from?

  • DavidR
    13 years ago

    " run the generator into a DC supply that you can parallel with the battery ..."

    I'd suggest using either a battery charger or a well-regulated power supply, set to the battery's float voltage or a bit above (around 13.8v for a 12v battery).

    The main downside of such a lashup is that it's not very efficient; there are losses in both charger and inverter. In general, the cheaper they are, the higher the losses. But it will work.

  • joed
    13 years ago

    If your running the generator why not just plug everything directly into that?

  • smithy123
    Original Author
    13 years ago

    i planned on running most of the lights off of the inverter/genset while running the generator only for the fog macine (500w) and 50 c9's @10w each and to take the load off of the inverter because with just the lights including the c9's, the alternator cannot keep up with the load. we woldd apperciate to run the generaytor only when we are moving.

  • smithy123
    Original Author
    13 years ago

    i could make an automatic transfer switch, so when the generator is running, i use the generator, but when the generator shuts off, i use the inverter. i am powering the 500w fogger, 3200w of c9s, a radio at times, 2 100w spotlights,and 100 mini lights. the battery only lasts 15-20 minutes even with the engine running the 100a alternator. i do not mind the gas consumption because the one paying for it owns a gas station, anyway, but i do not like the noise the whole time of a running generator.

  • brickeyee
    13 years ago

    "i do not like the noise the whole time of a running generator."

    That is what extension cords are for.

    A couple 50 foot #10s will get the generator 100 feet away with minimal voltage loss.

  • smithy123
    Original Author
    13 years ago

    it is on a parade float.

  • smithy123
    Original Author
    13 years ago

    btw, i plan building an extension cord with a 2 space qo panel in it, with an afci breaker.

  • joed
    13 years ago

    Add more batteries in parallel to be used during the parade. Recharge them after the parade is over.

  • smithy123
    Original Author
    13 years ago

    it would take alot of batteries to feed a 150a load. the auto transfer switch would be cheaper.

  • pharkus
    13 years ago

    ... something I've often thought of doing, but never have...

    and it's probably too late to propose such a kludge...

    take the generating part of a genset, shove it under the hood somewhere with a belt to the car engine...

    my original purpose for doing this was to run home audio gear (actually concert/pa stuff) in a van. :)

  • joed
    13 years ago

    150 amp generator is not going to fit under the hood.

  • Ron Natalie
    13 years ago

    150A at 12V can be done on an truck engine if you are clever enough. I've seen some 4WD RV things done that way.

    150A at 120V is going to take something the size of a car engine just to drive the generator.

  • brickeyee
    13 years ago

    You can get 120 V output alternators for vehicles.

    Creating very large low voltage currents is always a chore, and results in larger pieces of equipment.

    The typical car alternator is just below 100 amps at 12 V.

    It is actually a 3-phase AC alternator with a rectifier bridge to produce the DC.
    The battery takes car of filtering off the ripple.

  • DavidR
    13 years ago

    Probably 40-50 years ago I used to see magazine ads for kits to convert vehicle alternators to produce 120v. The illustration showed a guy with a power drill plugged into his car.

    Presumably they tweaked the field regulation (not so tough to do when regulators were external and mechanical) and bypassed the internal rectifiers. I'll bet the frequency was somewhere in close to the moon, though. Certainly it'd be tough to maintain 60 Hz. Universal-motor tools (most or all drills are) wouldn't care, but I suspect induction motor tools such as circular saws would throw fits.

    So I'd expect a similar problem with a modern alternator belted to your car or truck engine. You'd have to find some way to keep the RPM steady so the frequency would be correct, unless you borrowed the guts of an inverter-type genset.

    A plain, low-cost 12v inverter would be much easier to use - just connect (solidly) to the battery. Just be aware that if you exceed the alternator's capacity (~1000 watts or so unless you have a heavy duty vehicle), you'll drain the starting battery, so you can't do it for too long.

    It'll also really send your MPG into the cellar. Hmm, modern computer-controlled vehicle engines are generally more efficient than those relatively crude genset engines, but they're way oversized for the application. I wonder if an inverter setup would burn more or less fuel per kWh than a genset would.

  • pharkus
    13 years ago

    ? I know a 150A 120V generator won't fit under the hood...

    I also know the OP doesn't need 150A at 120V. In fact, I'm not sure where that idea even hatched from.

    I also know (relatively) the guts of an alternator. This video is one of my friends messing around with a "welder" we hacked up using one from a Toyota 22RE, belted to a motor scrapped from a mangled tablesaw. Removed the internal regulator and provided external excitation using the AC adapter from a cable modem. I'm the midget in the corner.


    Regarding frequency... our OP is running a bunch of lights (don't care about freq), a 'fogger' (if it's like the ones I've had, it's a heat load, which also doesn't care about freq), and a "radio"... the latter is really a question mark, since a small transformer-powered unit will care but not enough to matter, a larger transformer-powered one will definitely care, but some of the newer stuff is switchmode anyway. A device whose first step is to rectify to DC won't much care about the input frequency unless you're getting close to the switching speed limits of the rectifier diodes, which you are probably NOT going to do by driving a genset generator from a car engine at ANY realistic RPM... so really the detail of this "radio" is the only missing information to determine whether or not this hack would work.


    And I wasn't suggesting it as an actual solution anyway, merely stating, as a related sidenote, that I've always wanted to try it.

  • brickeyee
    13 years ago

    "So I'd expect a similar problem with a modern alternator belted to your car or truck engine. You'd have to find some way to keep the RPM steady so the frequency would be correct, unless you borrowed the guts of an inverter-type genset. "

    There are actually ways of controlling the field windings to maintain frequency.
    It is effective for slow variation, not as effective for quick speed changes.

  • pharkus
    13 years ago

    Wow, I never thought of that! (I do see how it would work though...)

  • smithy123
    Original Author
    13 years ago

    it's not my parents or family member's truck. it is the owner of a local gas station's, and he doesnt want to add a second alternator, etc. there is a 100a alternator, so it is 1,000 watts out of the inverter. the headlighrts do not turn off and the fogger also has an electric pump.

  • brickeyee
    13 years ago

    18,000 watts of lighting seems a little excessive.

  • pharkus
    13 years ago

    A little?

  • timrobins03
    6 years ago

    Alot of deep cycle batteries an an inverter that cuts off at 10.5 v

  • Ron Natalie
    6 years ago

    Wow, I'm sure after eight years Smithy was waiting around to hear that pearl of wisdom.


  • Henry C
    5 years ago

    Since someone is still responding... I have a 6500W portable gas generator with a 12VDC out socket. I want to hook up a 2000/4000 'car' inverter into the socket to run my laptop and electronics. DO I need a pure sine wave inverter or something special? Will the inverter keep the freqs in check? I know there are inverter generators but the one I have has a 7 gal gas tank and power for less $$. A decent inverter is $80. Another option is the UPS but that seems more expensive. Any ideas? THANKS!

  • DavidR
    5 years ago

    If the genset's DC output is reasonably stable and clean (no significant amount of ripple), and can produce enough current, any small 12v inverter should work fine on it.

    Note that I said "small" inverter. A typical ~70% efficient 2000 watt 12v inverter needs around 240 amps of DC for full continuous output, and double that to handle peaks. I doubt that your genset is designed to produce anywhere near that much DC. Even if it can, you won't get that kind of current through a lighter socket and plug.

    You might do OK with a 250 Watt inverter (around 25 amps). Probably 150 Watt would be safer.

    Whether you need a "pure sine" inverter depends on the electronics. So far I haven't had any problems running most wall warts and power pucks (either linear or switching type) on modified-sine (actually modified square wave) inverters.

    Don't try a laser printer though. They tend to be pretty fussy about the input waveform. I don't think a 250 Watt inverter could power one anyway.

  • Henry C
    5 years ago

    Wow thanks. I guess I should give the exact gen I am looking at. It's here dek 4550 Portable Generator It has both the socket and a direct 12V connection. I don't know enough about the internal specs but it seems to have a lot of power. I was hoping to run a 800W continuous inverter through it so I could power a laptop, printer, phone charger, etc like a small portable office in my cabin in the woods, for a few hours a day. Pure Sine Wave Inverter 800W

  • DavidR
    5 years ago

    After some searching I finally found specs for your genset here (pdf). It says the 12vdc output is 8.3 amps. With luck, that might handle a 100 watt inverter, but 70 watts (if you can find one that small) would be safer.

    Is there some reason you can't just plug your gear right into the genset's AC output?

    BTW, you might want to read the review (there's only one) on Amazon for that genset. Its 6500 watt stablemate also got mixed reviews.

    I recommend sticking with known brands for inverters, too. Both of the Chinese truck stop inverters I bought ~10 years ago failed after a few dozen hours of use well below rated power. Unlike some, mine produced no smoke or bangs, they just stopped working. I've had good luck with Statpower (now Xantrex).

  • Henry C
    5 years ago

    Are all the smart guys named Dave? I guess these inverter generators must have some heavy duty inverters. Can't just slap one on a generator and go. Sounds like I can get more watts out of my car battery.


    I guess I'm going to have to try something that will plug into generator AC and still protect my electronics. Maybe a really good power strip with surge protection, or power line conditioner, UPS, or other?


    Do you know of a good detailed guide for setting up a deep cycle battery bank off a generator that will then supply a 400W inverter?

  • DavidR
    5 years ago

    From what I understand, the big advantage that inverter gensets have over conventional ones is that they don't have to bang away at 3600rpm all day to keep the frequency stable. They can throttle up and down with the load, and thus use quite a bit less fuel.

    I've seen claims that inverter gensets produce cleaner power, but I don't see why that should be the case. In principle, the alternator in a conventional genset isn't dissimilar from what they run down at the power plant, just a few orders of magnitude smaller.

    Inverter gensets do have better frequency stability, but how many appliances really care any more? Most electronic gear today has switching power supplies. Usually the first thing the AC hits after the fuse and noise filter is a bridge rectifier, so frequency accuracy shouldn't be a big deal.

    A line conditioner wouldn't hurt anything but your wallet. I'd guess, though, that as long as your genset's voltage is reasonably stable, most modern computer and office equipment (other than a laser printer) isn't too likely to gripe about it.

    If anyone here has experience to the contrary, I hope he'll jump in here.

    Unless you're planning to go solar, I don't see much point to hauling a bunch of batteries up to your cabin just to charge from your genset. But if you do, the main issue is to make sure they're charged properly, not overcharged or undercharged, so they don't die an early death. A good smart (microprocessor controlled) charger is what you need. Many larger inverters have decent chargers built in.
    You might try contacting some of the online solar suppliers to see what they recommend. Or maybe we have some PV installers on the forum who have suggestions.

  • ionized_gw
    5 years ago

    Not much help here, but I find it a positive development that at least one very common manufacturer of gensets sized from portable and residential standby to very large is now making DC gensets geared to off grid electrical power. The need there is for backup in episodes of very poor weather and to equalize the batteries.

    I am not current in the technology, but previously there was no choice commonly offered other than running AC into the charge controllers (either as part of an inverter system or independent) to manage the batteries. That was supplied from POCO or your genset. You want a genset that runs quietly and efficiently at partial output? Buy an expensive inverter genset with a second inverter systems that you could do without.

    Now, you can probably build a very robust system with or without solar panels, inverters made for off-grid renewable power sources (PV, wind, hydro turbine) and a few batteries to handle startup surges and a DC genset. The genset may be automatic or run manually depending on how much you want to spend.